In concentration and collection of useful components, removal of unwanted components, fresh water generation and the like in fields of food industry field, medical field, electronic industry field and the like, a porous hollow fiber film which is made of cellulose acetate, polyacrylonitrile, polysulfone, fluorine-based resin and the like and is manufactured by a wet spinning method or a dry-wet spinning method, for example, is often used for a microfiltration film, ultrafiltration film, reverse osmosis filtration film and the like.
In a case of manufacturing the hollow fiber film by the wet spinning method or the dry-wet spinning method, an undiluted film forming solution containing a hydrophobic polymer and a hydrophilic polymer is prepared first. Subsequently, a coagulated substance, that is, a hollow fiber film precursor is formed through a film forming step of discharging this undiluted film forming solution in an annular shape and coagulating the same in a coagulated liquid. The undiluted film forming solution may be either introduced into the coagulated liquid via an air-contact traveling portion (dry-wet spinning method) or introduced directly into the coagulated liquid (wet spinning method).
Here, generally, in the film of the hollow fiber film precursor after the film forming step, the hydrophilic polymer remains a state of solution. If the hydrophilic polymer remains in the film in this manner, it is difficult for the hollow fiber film to exhibit high water permeability (water permeability in film).
Therefore, PTL 1 discloses that a hollow fiber film precursor after a film forming step is dipped into a low-temperature chemical containing an oxidizing agent such as sodium hypochlorite, and after causing the hollow fiber film precursor to maintain the chemical at a low temperature, the hollow fiber film precursor holding the chemical is heated in a gas phase, thereby decomposing the hydrophilic polymer remaining in the hollow fiber film precursor. After heating in the gas phase, a washing step of washing the hydrophilic polymer and a decomposition products thereof is performed.